class IteraterUtil {

   static List<T> change<T>(List<T?> list) {
    var result = <T>[];
    for (var item in list) {
      if (item != null) {
        result.add(item);
      }
    }
    return result;
  }

   static Set<T> changeSet<T>(Set<T?> list) {
     var result = Set<T>();
     for (var item in list) {
       if (item != null) {
         result.add(item);
       }
     }
     return result;
   }
}